When you’re a new user, you’re given a letter for your avatar.
You can easily customize that, and be a part of the in-crowd (yes, that was peer pressure 
) by visiting your preferences. Go ahead, click that link, it will take you there. The page looks like this.
Click the pencil, upload a photo, DONE.
See, easy peasy!
If you’re really daring, update your profile as well.